"Get the hell out of Dodge" is a reference to Dodge City, Kansas, which was a favorite location for westerns in the early to mid 20th century. Most memorably, the phrase was made famous by the TV show "Gunsmoke," in which villians were often commanded to "get the hell out of Dodge." The phrase took on its current meaning in the 1960s and 70s when teenagers began to use it in its current form.